1. Field of the Invention
The present invention relates to an information reproduction technique using a ferromagnetic material, and more particularly to a magnetic memory device utilizing a magnetoresistive effect element and a manufacturing method thereof.
2. Description of the Related Art
A magnetic random access memory (which will be abbreviated to an MRAM hereinafter) is a generic designation of solid-state memories which utilize a magnetization direction of a ferromagnetic material as a recording medium for information and are capable of rewriting, holding and reading recorded information at any time.
A memory cell of the MRAM usually has a structure in which a plurality of ferromagnetic materials are superimposed. Information is recorded by parallelizing or anti-parallelizing the relative arrangement of magnetization of a plurality of ferromagnetic materials constituting the memory cell, and associating the parallel or anti-parallel state with binary information “1” or “0”. Recorded information is written by passing a current to write lines arranged in the form of cross strips, and reversing the magnetization direction of the ferromagnetic materials in each cell. It is a non-volatile memory that power consumption when holding recorded information is zero in theory and recorded information is held even if a power supply is turned off. On the other hand, recorded information is read by utilizing a phenomenon that an electrical resistance of the memory cell varies depending on a relative angle between the magnetization direction of the ferromagnetic materials constituting the cell and a sense current or a relative angle of magnetization between a plurality of ferromagnetic layers, which is a so-called magnetoresistive effect.
Comparing functions of the MRAM with functions of a conventional semiconductor memory using a dielectric substance, the MRAM has many advantages. That is, for example, (1) the MRAM is completely non-volatile and rewriting for 1015 times or more is possible, (2) nondestructive reading is enabled and a refresh operation is not required, thereby shortening a read cycle, and (3) the resistance against radiation rays is strong as compared with a charge storage type memory cell, and others. It is predicted that a degree of integration per unit area and write and read times of the MRAM can be roughly the same as those of a DRAM. Exploiting the great characteristic of non-volatility, therefore, application to an external memory device for a portable device, a use with an LSI and a main storage memory in a personal computer is expected.
At present, the MRAM which has been examined to be put into practical use employs an element which demonstrates a tunneling magnetoresistive effect (which will be abbreviated to a TMR effect hereunder) for the memory cell (for example, see a non-patent cited reference 1). The element demonstrating the TMR effect (which will be referred to as an MTJ (Magnetic Tunneling Junction) element hereinafter) is mainly formed by a three-layer structure consisting of a ferromagnetic layer/an insulating layer/a ferromagnetic layer, and a current flows with the insulating layer being tunneled. A tunnel resistance value varies in proportion to a cosine of a relative angle of magnetizations of the both ferromagnetic metal layers, and takes a local maximum value when the both magnetizations are anti-parallel. At a tunnel junction consisting of, e.g., NiFe/Co/Al2O3/Co/NiFe, a magnetoresistive change ratio exceeding 25% is found in a low magnetic field not more than 50 Oe (see, e.g., a non-patent cited reference 2). As a structure of the MTJ element, there are known a so-called spin valve structure in which an antiferromagnetic material is arranged in contiguity with one ferromagnetic material and the magnetization directions are fixed for the purpose of improving the field sensitivity (see, e.g., a non-patent cited reference 3), and a structure that a double tunnel barrier is provided in order to improve the bias dependence of a magnetoresistive change rate (see, e.g., a non-patent cited reference 4).
When applying the MTJ element to the MRAM, electrodes at both ends of the MTJ element must be connected to data lines and an external circuit such as a selection transistor or the like. In particular, since the MTJ element has a vertical structure, the element separation must be carried out by using the insulating film when connecting the upper electrode on the MTJ element to an external wiring. As this insulating film, a contact hole for wiring connection is formed. As a method of forming the contact hole, there are mainly used two methods, i.e., (1) a method of using a resist mask and etching the insulating film by reactive chemical etching or the like, and (2) a method of forming the insulating film while leaving a resist used in element processing and then peeling the resist by using a solvent or the like (self-alignment process).
The method (1), however, has a drawback that a mask alignment margin in the above-described process defines a minimum processing dimension and it is difficult to realize minuteness, and other drawbacks. Further, the method (2) has a disadvantage that peeling of the resist is difficult when realization of minuteness advances and a thickness of the photoresist becomes approximately equal to an element dimension. It is to be noted that a patent cited reference 1 discloses a method of depositing an insulating film on the entire surface after processing the element, then performing etch-back to the element surface and opening the contact in the self-alignment manner as a manufacturing method realized by improving the method (2).
[Non-Patent Cited Reference 1]
Roy Scheuerlein, et al., A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each cell, “2000 ISSCC Digest of Technical Papers”, (USA), February 2000, p. 128-129
[Non-Patent Cited Reference 2]
M Sato, et al., Spin-Valve-Like Properties and Annealing Effect in Ferromagnetic Tunnel Junctions, “IEEE Trans. Mag.”, (USA), 1997, Vol. 33, No. 5, p. 3553-3555
[Non-Patent Cited Reference 3]
M Sato, et al., Spin-Valve-Like Properties of Ferromagnetic Tunnel Junctions, “Jpn. J. Appl. Phys”, 1997, Vol. 36, Part 2, p. 200-201
[Non-Patent Cited Reference 4]
K Inomata, et al., Spin-dependent tunneling between a soft ferromagnetic layer and hard magnetic nano particles, “Jpn. J. Appl. Phys.”, 1997, Vol. 36, Part 2, p. 1380-1383
[Patent Cited Reference 1]
Specification of U.S. Pat. No. 5,841,692
A concrete example when the MTJ element is applied to the MRAM will now be described with reference to the accompanying drawings.
FIG. 27A is a plane view showing a magnetic memory device according to a prior art. FIG. 27B is a cross-sectional view of the magnetic memory device taken along the line XXVIIB—XXVIIB in FIG. 27A. FIGS. 28A and 28B are cross-sectional views of a magnetic memory device including a memory cell portion (which will be referred to as a cell portion hereinafter) of the magnetic memory device according to the prior art and a peripheral circuit portion (which will be referred to as a core portion hereinafter) of the magnetic memory device. In the cell portion, as shown in FIG. 28A, a magnetoresistive effect element 14a is arranged on a lower metal layer 13a, and the magnetoresistive effect element 14a is connected to a selection transistor 3a through the lower metal layer 13a and a contact 12. On the other hand, a magnetoresistive effect element and a lower metal layer are not formed in the core portion as shown in FIG. 28B.
FIGS. 29A and 29B to FIGS. 38A and 38B show a method of manufacturing a cell portion and a core portion of a magnetic memory device illustrated in FIGS. 28A and 28B. Here, the respective figures A show the cell portion in the magnetic memory device, and the respective figures B show the core portion in the magnetic memory device. A method of manufacturing the memory device according to the prior art will now be described. It is to be noted that description will be first given as to a process after forming a lower contact 12 in a first insulating film 11.
As shown in FIGS. 29A and 29B, a lower metal layer 13 is first formed on a first insulating film 11 in common with the cell portion and the core portion, and a magnetoresistive effect film 14 is formed on the lower metal layer 13. Then, first and second hard masks 15 and 16 are superimposed on the magnetoresistive effect film 14. Here, the first hard mask 15 consists of, e.g., a conductive film, and the second hard mask 16 is made up of, e.g., a non-conductive film.
Then, as shown in FIGS. 30A and 30B, the second hard mask 16 is selectively etched in common with the cell portion and the core portion. As a result, the second hard mask 16 to which a shape of the magnetoresistive effect element has been transferred remains in the cell portion, but the second hard mask 16 is all removed in the core portion.
Subsequently, as shown in FIGS. 31A and 31B, the first hard mask 15 is etched in common with the cell portion and the core portion. As a result, since etching is effected by using the second hard mask 16, the first hard mask 15 to which a shape of the magnetoresistive effect element has been transferred remains in the cell portion, but the first hard mask 15 is all removed in the core portion.
Thereafter, as shown in FIGS. 32A and 32B, the second hard mask 16 is peeled in the cell portion.
Then, as shown in FIGS. 33A and 33B, in common with the cell portion and the core portion, a magnetoresistive effect film 14 is etched. Consequently, since etching is effected by using the first hard mask 15, a magnetoresistive effect element 14a consisting of the magnetoresistive effect film 14 is formed in the cell portion, and the magnetoresistive effect film 14 is all removed in the core portion.
Subsequently, as shown in FIGS. 34A and 34B, in common with the cell portion and the core portion, a photoresist 18 is applied and patterned. As a result, the photoresist 18 patterned into a shape of the lower metal layer 13 remains in the cell portion, but the photoresist 18 is all removed in the core portion.
Next, as shown in FIGS. 35A and 35B, in common with the cell portion and the core portion, the lower metal layer 13 is etched. Consequently, since etching is effected by using the photoresist 18, the patterned lower metal layer 13 remains in the cell portion, but the lower metal layer 13 is all removed in the core portion. Thereafter, the photoresist 18 is removed in the cell portion.
Then, as shown in FIGS. 36A and 36B, in common with the cell portion and the core portion, a second insulating film 21 is formed on the entire surface.
Subsequently, as shown in FIGS. 37A and 37B, the second insulating film 21 in each of the cell portion and the core portion is etched back by using, e.g., chemical mechanical polish (which will be abbreviated to CMP hereinafter). As a result, in the cell portion, the surface of a contact 15a consisting of the first hard mask 15 is exposed, and a contact is opened in the self-alignment manner.
In the above-described prior art process, however, when etch-back is carried out after forming the second insulating film 21, the following problems occur.
As apparent from FIGS. 36A and 36B, although the lower metal layer 13, the magnetoresistive effect element 14a and the first hard mask 15 exist in the cell portion, these patterns do not exist in the core portion. Therefore, since the cell portion and the core portion are different from each other in unevenly pattern density (which will be abbreviated to flatness hereinafter), there occurs a difference in a polishing speed between the cell portion and the core portion when etching back the second insulating film 21.
As described above, since the polishing speed of CMP varies depending on the flatness, the second insulating film 21 in the core portion may remain with a small thickness (see FIG. 37B) or a large thickness (see FIG. 38B) in some cases as compared with the second insulating film 21 in the cell portion.
In particular, as shown in FIGS. 37A and 37B, when the insulating film 21 becomes thin in the core portion as compared with the cell portion, the lower wiring (not shown) may be exposed in some cases, which provokes a problem of the short circuit and the like of the wiring.
Further, in the conventional process, there is no appropriate end point detection method with respect to etch-back. In case of CMP, since the polishing speed is lower with a slurry and under polishing conditions used in polishing of the second insulating film 21, an end point can be detected by using known end point detecting means such as stress detection based on the first hard mask 15 consisting of a metal on the magnetoresistive effect element 14a. However, since the first hard mask 15 exists only in the cell portion, its area occupying in the whole is small, and hence the end point detection sensitivity is deteriorated. As a result, since the polishing speed varies in the cell portion in which the surface of the hard mask 15 is exposed, it can be expected that a large difference in a polishing quantity can be generated between the cell portion and the core portion.
Although description has been given herein as to the method of exposing the contact 15a by using CMP, unevenness of the contact exposure or difficulty in detecting an end point which relies on the original flatness is involved even in, e.g., a method of etching back the entire surface after flattening the whole by using the flattening film, and the same problem as that in case of CMP occurs.